1. Field of the Invention
This invention relates to thermal ink jet printing, and more particularly to a printhead array and methods of fabricating butted printhead arrays for improving print quality in pagewidth array printheads having a plurality of butted printhead die assemblies.
2. Description of Related Art
Thermal ink jet printheads typically include a heater plate that includes a plurality of resistive heating elements and passivated addressing electrodes formed on an upper surface thereof and a channel plate having a plurality of channels, which correspond in number and position to the heating elements, formed on a lower surface thereof. The upper surface of the heater plate is bonded to the lower surface of the channel plate so that a heater element is located in each channel. The channel plate usually includes at least one fill hole extending from its upper surface to its lower surface that is in direct fluid communication with the channels so that ink is supplied from a source into the channels.
Drop-on-demand thermal ink jet printheads typically are fabricated by using silicon wafers and processing technology to make multiple small heater plates and channel plates. This works extremely well for small printheads. However, for large array or pagewidth printheads, a monolithic array of ink channels cannot be practically fabricated in a single wafer since the maximum commercial wafer size is six inches. Even if ten inch wafers were commercially available, it is not clear that a monolithic channel array would be very feasible. This is because a single defective channel out of 2,550 channels would render the entire channel plate useless. This yield problem is aggravated by the fact that the larger the silicon ingot diameter, the more difficult it is to make it defect-free. Furthermore, most of the wafer would be thrown away, resulting in very high fabrication costs.
Since silicon wafers are not currently available having a length corresponding to a pagewidth, the current practice is to form the nozzles, passageways and integrated circuitry on silicon wafers, separate these wafers into wafer subunits (or chips) which contain butt surfaces or edges, align these subunits along their butt surfaces or edges into an array having a length of a pagewidth, for example, and attach the array to a substrate to form a pagewidth printhead. The layering of the wafers (i.e., the channel wafer and the circuitry wafer), if necessary, to form the complete printhead can be performed before or after separation into subunits. Since many wafer subunits are aligned to form an array, each subunit must be uniform. In order for the subunits to be uniform, the location of the butt edges or surfaces relative to the circuitry and channels must be precise.
Discrete printheads may be fabricated by forming a plurality of sets of heating elements and a plurality of sets of channels in separate silicon wafers that are later bonded to each other and separated, such as by dicing, to form discrete printhead modules (or die assemblies). The sets of heater elements and sets of channels are located on their respective silicon wafers in a plurality of rows and columns to form corresponding matrices thereon. The bonded wafers are separated between each row and column to form the discrete printhead modules. Each discrete printhead module includes a portion of the wafer containing the heater elements (known as a heater plate) and a portion of the other wafer containing a set of channels (known as a channel plate). After forming the discrete printhead modules, a plurality of the printhead modules can be aligned and butted against one another on a support substrate, such as, for example, a heat sink, to form a pagewidth printhead formed from a linear array of printhead modules.
In an attempt to improve alignment in a pagewidth array, Drake et al. U.S. Pat. No. 4,829,324 discloses a large array thermal ink jet printhead. The printhead is formed of an array of abutting individual subunits. Each subunit includes etched sloping sides that permit accurate alignment of adjacent subunits. In a separate embodiment, the sloping sides are produced by dicing along a large etched groove formed on one end of the channel plate.
U.S. Pat. No. 4,829,324 does not recognize that grooves formed adjacent to the end channels of each channel array (i.e., each channel plate) in the surface containing the channels can protect the end channels from damage caused by cracking that occurs during dicing. Referring to FIG. 12 of U.S. Pat. No. 4,829,324, it can be seen that the dice cut on the left side of each subunit is spaced a relatively large distance from the left-most channel and therefore would not affect the left-most channel even if cracking were to occur. A large groove formed from the surface of the channel wafer opposite from the channel surface defines the right side of each subunit. Accordingly no dicing is performed on the right side of the channel plate. Additionally, etched surfaces define the butt ends of each subunit, rather than diced surfaces. The formation of the large etched grooves is time consuming, thereby increasing production time and costs.
Drake et al. U.S. Pat. No. 4,961,821 discloses a method of fabricating a pagewidth printhead for an ink jet printing device having a plurality of abutted individual subunits. The individual subunits are formed without dicing. Subunits are separated from each other by anisotropically etching first and second intersecting recesses.
Ormond et al. U.S. Pat. No. 5,128,282 discloses a process for separating image sensor dies from a wafer that minimizes silicon waste. Each row of dies on a wafer is separated by a pair of separation V-grooves. The grooves are provided to reduce microscopic damage occurring in the die surface during a dicing operation. The provision of the grooves reduces damage to the active surface of the dies and any circuits contained thereon. A related process also is disclosed in Jedlicka et al. U.S. Pat. No. 4,814,296.
Fisher et al. U.S. Pat. No. 5,160,403 discloses a method of fabricating a printhead die having a buttable surface. A pagewidth printhead is formed from a staggered array of discrete ink jet print modules. Each module is manufactured by providing a shallow precision dice cut that defines a lateral aligning surface having a minimal height in the surface of a channel plate defining substrate adjacent to each of the channels.
Another related patent is Campanelli et al. U.S. Pat. No. 4,786,357. Grooves are provided for dicing between individual die assemblies. Dicing is spaced a sufficient distance from the end channels of the die assemblies such that end channel damage is not a concern. Additionally, the dice cuts do not form buttable surfaces.
In all ink jet printing systems, the nozzle or channel size, shape and surface conditions affect the characteristics and trajectory of the ink droplet emitted from the channel. All of these factors affect print quality. The prior art discussed above does not address the impact of dicing the channel plate during formation of individual printheads on print quality. As the channel plate is diced into individual printheads, end channels can be subject to chipping as a result of the dicing process when the dicing is performed very close to the end channels, as is desired in some printhead arrangements. The chipping can affect the size, shape and surface conditions of the end channels. This adversely affects the overall print quality.